This invention relates to controlling the transfer of energy between a current-mode storage device and a load by controlling a phase angle of power delivered to the load.
Energy storage devices, such as a superconducting magnet, have been contemplated to provide power to a utility network to compensate for power shortfalls in the network. For example, large superconducting magnetic energy storage (xe2x80x9cSMESxe2x80x9d) systems have been proposed for diurnal energy storage. In such devices, energy is transferred from a utility network to the storage device when the utility demand is low (e.g., at night) and energy is transferred from the storage device to the utility network when the demand is high (e.g., summer afternoons).
Energy transfers between a current-mode storage device and a load (such as a utility network) are more efficient when a voltage across output terminals of the current-mode storage device is constant. Such energy transfers are referred to as xe2x80x9cconstant voltagexe2x80x9d transfers.
The invention features a method of controlling a transfer of power between an energy storage device and a load. In general, in one aspect, the method includes obtaining a DC voltage from the energy storage device, and controlling a phase angle of AC power delivered to the load to keep the DC voltage substantially constant. By controlling the phase angle to keep the DC voltage substantially constant, constant voltage energy transfers can be performed more efficiently without complex switching mechanisms or additional hardware, such as hysteretic controllers or storage cells.
This aspect of the invention may include one or more of the following features. The energy storage device includes a current-mode energy storage device, such as a superconducting magnet. The transfer of power includes discharging power from the energy storage device to the load. The method maintains the DC voltage substantially constant by controlling a current component of the AC power. The method controls the current component of the AC power to keep an output voltage component of the AC power substantially constant. Thus, the invention can be used to meet the constant voltage requirements of many utilities.
Discharging the magnet at a substantially constant voltage decreases discharging time (relative to constant power or constant resistance discharge). As a result, the likelihood (and/or amount) of damage to the magnet can be reduced. Also, if the magnet is connected to a utility network, for example through an inverter, discharging the magnet at a substantially constant voltage increases the rate at which power can be supplied to the utility network. As a result, the utility network can be stabilized more quickly following a fault.
The method may be performed using an inverter. Power losses in the inverter may be compensated by controlling the phase angle of the AC power. Compensating for power losses in the inverter includes determining a difference between voltage at input terminals of the inverter and a preset voltage value, and changing the phase angle of the AC power delivered by the inverter in accordance with the difference.
Other advantages and features of the invention will become apparent from the following description and claims.